Idiopathic thrombocytopenic purpura (ITP) results from an immunologic reaction against an autologous platelet-associated antigen(s) with subsequent antiplatelet antibody production with resultant platelet sensitization and phagocytosis. The present proposal has two goals: (1) To apply the technics developed thus far by us for evaluating the origin and magnitude of production of antiplatelet antibody to the study of a large series of ITP patients. (2) Develop new methods to better evaluate the platelet antigen(s) involved and the secondary events which follow the binding of antibody to platelets. IgG production by cells from the spleen, marrow, and nodes from ITP patients will be measured using the Fab anti Fab assay system and the percentage of platelet-specific IgG will be determined. The total quantity of platelet-specific IgG will be calculated by correcting for the number of cells per organ and the amount required to saturate the antigen sites will also be measured. In addition, the IgG associated with the ITP platelets in vivo will be quantitated. These data will be correlated with the patient's clinical course. Platelet antigens will be studied by reacting radiolabeled platelet surface proteins with known antiplatelet antibody and evaluating the fractions precipitated by antihuman IgG. In addition, the degree of chemotaxis, fixation of complement and platelet phagocytosis resulting from the antibody-antigen interaction will be investigated.